if your stats are. Used 20 ,,, kills 5 --> means that piece of equipment gains you a kill 25% of the time you use it. So if an unequipped item has stats of Used 40,,, kills 20. --> you should be using that one as your getting kills 50% of the time.

You shouldn't always pick a weapon based on your current performance numbers with it. Call of Duty is also about feel, and not just about numbers. When I started off with the AK-12, I had a negative weapon KD, but I continued using it and now, I always go positive with it and the weapon KD changed.